Bacterial infections of the pregnant uterus are major causes of preterm birth, stillbirth, maternal death, and childhood disability. Group B Streptococcus (GBS) is a major culprit, but advancing preventive and therapeutic strategies has been slowed by gaps in our understanding of the host-microbial interactions responsible for infection. Our long term vision is to reduce the burden of perinatal infections by defining molecular mechanisms of disease pathogenesis. The goal of this project is to newly identify host and bacterial determinants of invasive GBS infection. There is considerable variation in the ability of phylogenetically distinct GBS strains to cause disease in neonates and pregnant women, ranging from asymptomatic colonization to severe infection. The basis of these differences is unclear. Early stages in the interaction between GBS and maternal gestational tissues potentially determine the outcome of infection. New data suggest that both host and bacterial responses to initial contact vary significantly depending on the strain, yet this has received almost no attention in regards to maternal-fetal infection. Based on preliminary data from our groups and others, we hypothesize that (1) distinct virulence gene expression profiles in unique multilocus sequence types (STs) of GBS are triggered by contact with human gestational tissues, and (2) host immune responses to GBS are shaped by the virulence of the infecting ST. We propose to test these novel hypotheses through three Specific Aims that take advantage of collaborations among experts in microbial pathogenesis, reproductive biology, infectious diseases, and innate immunity. The Aims are: (1) identify differences in global transcriptome remodeling of diverse GBS STs during infection of human placental macrophages and intact extraplacental gestational membranes; (2) define host immune responses to diverse GBS sequence types that mediate invasive vs. colonizing GBS phenotypes in human extraplacental gestational membranes; and (3) determine the extent to which placental macrophage behavior is influenced by GBS ST variation. These studies will shed new light on the pathogenesis of perinatal GBS infections, revealing novel targets that could be used to improve the health of mothers and children worldwide.